Gas turbine engines are known to comprise a compressor, one or more combustion chambers and one or more turbines. The compressor supplies compressed air to the combustion chambers wherein a fuel is injected and combusted, generating hot gases that are expanded in the turbines, to gather mechanical work.
Typically a combustion chamber has an annular combustion device connected to nozzles that supply fuel into it (diffusion combustion chamber) or mixing devices that supply a mixture of air and fuel into it.
One of the key factors for the gas turbine engine operation is the flame temperature, i.e. the temperature of the flame within the combustion chamber.
In fact, if the flame temperature is too high the NOx emissions are high, and if the flame temperature is too low, pulsations are generated within the combustion chamber; for these reasons, during operation, the flame temperature must be within a given range that allows correct operation.
In addition, when operating within the given range, the gas turbine engine operation could also be troubling.
In fact, possible pulsations generated by a single nozzle or a single mixing device are generally influenced and influence the pulsations of adjacent nozzles or mixing devices.
For this reason it is possible that, even when operating within the given range, pulsations are naturally generated at single nozzles or mixing devices (e.g. for mechanical defects or tolerances, particular local conditions, etc) and couple with pulsations of adjacent devices, generating a rotating pulsation wave within the combustion chamber.
These rotating pulsation waves are very detrimental for the gas turbine engine lifetime and must be damped.
In order to damp these rotating pulsation waves, traditionally throttling of fuel to selected nozzles or mixing devices is implemented. Because of this throttling, the selected nozzles or mixing devices generate a flame with a temperature that is lower than the temperature of the flame generated by the other nozzles or mixing devices; in other words, the flame temperature distribution within the annular combustion chamber is uneven, this having a beneficial effect on damping the rotating pulsation wave.
Usually, throttling is achieved by using orifices having a fixed diameter that are installed in the combustor fuel inlet pipe of selected burners. These orifices reduce the fuel flow through the pipe causing a reduced amount of fuel to be injected within the combustion chamber at the selected locations and thus the described reduced flame temperature.
Nevertheless, even if they allow damping of the rotating pulsation wave, the orifices introduce different constraints.
In fact, regulation and optimization of the fuel supplied to nozzles or mixing devices require the replacement of the orifices and are therefore very time consuming; for example the engine must be stopped and re-adjusted every time the orifices are replaced.
In addition, online regulation according to the different operating conditions (and as a result, flame temperature and pulsation level) is not possible.